1. Field of the Invention
This invention relates to liquid crystal devices and is particularly concerned with liquid crystal devices in which the liquid crystal material can be selectively controlled by the application of an electric or magnetic field to vary the light transmissivity of the device, as in an optical shutter for example.
2. Description of the Prior Art
It is known from, for example, French Patent No 2139537, U.S. Pat. No. 4,435,047 and U.S. Pat. No. 4,671,618 to encapsulate a nematic liquid crystal material as a multiplicity of droplets within a matrix of transparent, encapsulating material across which an electric or magnetic field can be selectively applied. In the field-off state, the nematic liquid crystal molecules in each capsule tend to align with the three dimensional boundary wall of the encapsulating material bounding the capsule so that collectively, the liquid crystal molecules d-o not have any preferred overall direction of alignment. In this condition, the liquid crystal material is insensitive to the polarization of light incident on the device and the device is then substantially opaque.
When a field of suitable magnitude is applied to the device, the tendency of the liquid crystal molecules to align with the boundary walls of the capsules is overcome and the molecules under the influence of the applied field tend to re-orientate into generally parallel alignment and in this condition light can be transmitted through the device.
It is also known from U.S. Pat. No. 4,411,495 that selective light transmission through a liquid crystal device can be effected by dispersing a liquid crystal material within a layer comprising a porous filter material made of mixed esters of cellulose and controlling the refractive index of the liquid crystal No. 4,411,495. material by the application of an electric field to produce conditions in which the refractive indices of the liquid crystal and filter material are matched or mismatched. When the refractive indices are matched, the device is transmissive and, when they are mismatched, the device becomes opaque to incident light. It will be noted that refractive index changes are fundamental to the operability of devices constructed in accordance with U.S. Pat. No 4411495.